1. Field of the Invention
The present invention relates to displays including organic electroluminescent devices (herein referred to as organic EL devices) and particularly relates to an organic electroluminescent display (herein referred to as an organic EL display) having high quality display.
2. Description of the Related Art
An organic EL device is a device that includes an anode, a cathode, and a layer disposed therebetween which contains a fluorescent organic compound and which emits light according to the following mechanism: holes and electrons are injected from the anode and the cathode, respectively, into the layer such that excitons of the fluorescent organic compound are formed, whereby luminescence is achieved when the excitons returns to the ground state.
According to a study by Eastman Kodak Co. (Appl. Phys. Lett., vol. 51, pp. 913-915 (1987)), it has been reported that a function-separation type organic luminescent device has a luminance of about 1,000 cd/m2 under the application of a voltage of 10 volts; such a device includes an anode containing ITO, a cathode containing a magnesium-silver alloy, a first layer containing tris(8-quinolinolato) aluminum having an electron-transporting function and a luminescent function, and a second layer containing a triphenylamine derivative having a hole-transporting function. Examples of related U.S. patents include U.S. Pat. Nos. 4,539,507, 4,720,432, and 4,885,211.
Furthermore, by changing species of the fluorescent organic compound, it is possible to effect luminescence in a broad wavelength region ranging from the ultraviolet region to the infrared region. In this regard, various compounds have been extensively studied in recent years. Such compounds are disclosed in, e.g., U.S. Pat. Nos. 5,151,629, 5,409,783, and 5,382,477, and Japanese Unexamined Patent Application Publication Nos. 2-247278 (corresponding to U.S. Pat. Nos. 5,130,603 and 6,093,864), 3-255190 (corresponding to U.S. Pat. No. 5,227,252), 5-202356, 9-202878, and 9-227576.
In addition to the above-mentioned organic EL device, another organic EL device using a conjugated polymer has been reported by a research group at Cambridge University (Nature, vol. 347, pp. 539-541 (1990)). According to this report, a layer containing polyphenylenevinylene (PPV) is formed with a coating apparatus, and luminescence is achieved by the use of such a single layer. Such an organic luminescence device using a conjugated polymer is disclosed in, e.g., U.S. Pat. Nos. 5,247,190, 5,514,878, and 5,672,678, and Japanese Unexamined Patent Application Publication Nos. 4-145192 (corresponding to U.S. Pat. No. 5,317,169) and 5-247460.
As described above, recent progress with respect to organic EL devices is noticeable; that is, organic EL devices have high luminescence and high speed responsiveness under the application of a small voltage and also have a small thickness and weight, thereby allowing such devices to be used in various applications. In particular, the use of the devices in display applications has become a focus of attention.
FIG. 5 is a sectional view showing a conventional organic EL device. This device includes a first electrode 1 that is transparent, a second electrode 2 that is reflective, and an organic compound layer 3 disposed therebetween. In the device, light emitted from the organic compound layer 3 is transmitted through a light-transmitting surface disposed on the side close to the first electrode 1. Since external light 4 entering the light-transmitting surface is reflected by the second electrode 2 and then transmitted through the light-transmitting surface, the external light 4 is mixed with the light emitted from the organic compound layer 3. Therefore, in organic EL displays including such organic EL devices arranged in a two-dimensional manner, there is a problem in that the contrast is low, and thus the display quality is poor.
Furthermore, in the luminescence of organic EL devices, the monochromaticity and color purity are insufficient for some uses. In order to solve this problem, an organic EL device having a microresonator structure (microcavity structure) has been proposed. Such a device is disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 6-275381 (corresponding to U.S. Pat. No. 5,554,911) and 11-288786 (corresponding to U.S. Pat. No. 6,406,801). According to this configuration, in the luminescence of the device, the directivity is high in the direction facing the front face of the device, and the monochromaticity is high depending on the resonant frequency of a microresonator.
FIG. 6 is a sectional view showing a conventional organic EL device having microresonator structures. This device includes a first electrode 5 that is transparent, a second electrode 6 that is reflective, an organic compound layer 7 disposed therebetween, and a multi-layer dielectric mirror layer 8 disposed on the first electrode 5.
Light emitted from the organic compound layer 7 resonates in a resonator formed by the multi-layer dielectric mirror layer 8 and the second electrode 6, whereby a first ray 9a having the following features is emitted: directivity that is high in the direction facing the front face of the device and high monochromaticity, that is, a particular wavelength band. However, in a second ray 9b emitted in a direction diagonal to the organic EL device, the frequency is shifted.
FIG. 7 is a graph showing the relationship between the observation angle (that is, an emission angle) and the peak of luminescence wavelength. As shown in FIG. 7, an increase in observation angle shifts the wavelength of observed light to a short wavelength region. In an organic EL display including the organic EL devices arranged in a two-dimensional manner, each having the microresonator structure, although the color purity of emitted light is excellent, the color of the emitted light observed in a direction diagonal to the organic EL devices is different from that of the emitted light observed in the direction facing the devices. Therefore, there is a problem in that the color purity is deteriorated depending on the observation position, that is, the display quality is deteriorated.